Pumps and compressors operated in the recovery of hydrocarbon products at great sea depths are subjected to a harsh environment including e.g. high pressures in the order of well over 100 bar. In order to prevent ingress of liquid, gas and solid particles into the motor and pump/compressor structures, hydraulic fluid at slightly higher pressure is circulated through the structures to provide lubrication and barriers that prevent intrusion of harmful matter. Typically, barrier and lubrication fluid is circulated through the motor structure at a first pressure, and barrier and lubrication fluid is circulated through the pump or compressor structure at a pressure that is slightly lower than the pressure in the motor fluid circuit. A pressure difference of about 5-10 bar is often applied and sufficient to separate the motor and pump/compressor fluid circuits, and the same pressure difference can be applied to separate the barrier and lubrication fluid circuits from the production fluid in the pump or compressor.
The motors, pumps or compressors used for this purpose are heavy duty designs requiring considerable power to bring the pump's or compressor's rotor(s) in rotation, as well as long life bearings to take up thrust load applied to the rotor by its weight and from the production fluid. Once in rotation lubrication fluid is pressurized and fed by the rotor or by means associated with the rotor to supply tilted pads of a thrust bearing to allow for hydrodynamic lubrication film build-up and thereby to keep thrust-engageable surfaces separated with a fluid film flow between the surfaces.
At stand still, the thrust-engageable surfaces may come into contact under the weight of the rotor, e.g., or in result of the static pressure in the pumped medium at stand still. Changeover from rotation to non-rotation and vice versa, i.e. at start-up and shut-down, imposes a problem as the hydrodynamic operation of the thrust bearing fails when the rotor speed is insufficient for separating the thrust-engageable surfaces. At start-up and shut-down, rotation of the “dry” bearing causes wear of the thrust-engageable surfaces. Start-up of a pump or a compressor with dry bearings results initially in a very high torque demand, which calls for a corresponding over-sizing of the subsea power distribution grid, motor and transformers, etc.